Electric arc plasma burner



July 15, 1969 Filed April 8, 1966 United States Patent 3,456,146ELECTRIC ARC PLASMA BURNER Giinther Hess, Erlangen, Germany, assignor toSiemens Aktiengesellschaft, Erlangen, Germany, a corporation of GermanyFiled Apr. 8, 1966, Ser. No. 541,223

Claims priority, application Germany, Apr. 12, 1965,

Int. Cl. HOlj 61/28 U.S. Cl. 313231 2 Claims ABSTRACT OF THE DISCLOSUREMy invention relates to plasma burner for heating gases in an electricarc chamber wherein a burning are located between two electrodes isrotated in a magnetic field.

In plasma burners of this type, a continuous gas flow is heated in anelectric arc so that the enthalpy of the gas is increased. With the heatenergy absorbed by the gas, high flow velocities can be achieved withthe aid of nozzles for use in wind tunnels and jet propulsion where suchhigh fiow velocities are required, or for carrying out chemicalreactions at high gas temperatures.

Plasma burners with a rotating are are particularly suitable fortransforming a high power from the heated plasma to electricity becausethe load on the electrode caused by the electric arc is distributed overa large surface. The electrodes can be cooled continuously to such anextent that the foot or base of the electric arc does not burn into orcause vaporization of the respective electrode so that the material ofthat electrode is thereby protected against being consumed by suchvaporization. Rotating arcs are primarily employable when electrodes ofheavy metal such a tungsten are not installed in a plasma burner inwhich the working gas is highly aggressive, but rather metals of lowermelting points such as copper are employed as the electrodes. Anadditional advantage of plasma burners having rotating arcs, besidesthat of being able to use electrodes of lower mel ing point, is that thearc passes through a large quantity of gas and, due to the turbulenceproduced, simultaneously heats up to some extent.

The rotation of the arc is carried out in an applied magnetic fieldwhose field lines extend in a direction perpendicular to the are. In aheretofore known conventional plasma burner, the arc burns between arod-shaped electrode and an annular electrode located coaxial to therod-shaped electrode so that the arc extends in a radial directionbetween the electrodes. In this known plasma burner, the electrodes arecentrally located in a cylindrical vessel on which a field coil is woundconcentrical to the common axis of the electrodes. A disadvantage ofthis known plasma burner is that only a small part of the magnetic fieldcontributes to the rotation of the arc. Moreover, because of the greatdifficulty in providing effective cooling, a yoke for concentrating themagnetic "ice field lines cannot be installed due to the lack ofsufficient space as well as unsuitable geometry.

There has also been suggested heretofore that the are be produced in amagnetic cusp field between cylindrical electrodes of the same sizelocated along a common axis. Symmetrical proportions and characteristicsare thereby produced for cooling the electrodes. However, thedisadvantage must then be taken into consideration that the strongcentral field of the coils cannot be utilized but rather only theportions of the weaker marginal field having radially extending fieldlines. The production of the magnetic field is consequently considerablymore expensive.

In order to increase the heating effect of the plasma burners, it hasalso been suggested heretofore to construct an arc chamber wall of astack of ring-shaped electrodes and insulating rings lying therebetween,which are provided with coolant and gas channels. Gas distributionchannels are formed in the stacked cylindrical wall of the arc chamberfor supplying the working gas between the electrodes. Several arcs canthen be formed extending in the axial direction between pairs ofring-shaped electrodes. With this known arrangement, however, thedisadvantages of the cusp field must also be taken into consideration.

It is accordingly an object of my invention to provide a plasma burnerwhich avoids the disadvantages of the aforementioned known plasmaburners, particularly with regard to the formation of the magneticfield, but without reducing the advantages derived from such knownplasma burners.

It is, furthermore, an object of my invention to provide a plasma burnerwhich utilizes the central or core field of the ring-shaped coils forproducing the rotation of the arc and, however, supplying the magneticfield through pole shoes to the arc in a beam or narrow bundle.

It is also an additional object of my invention to provide a plasmaburner having excitation windings and yoke that are easily cooled.

It is yet another object of my invention to provide a plasma burnerwherein the length of the arc and the transformed power therewith can beadjusted to the gas throughput; in addition to widening or broadeningthe are for greater gas throughput, the length of the arc iscontrollable by adjusting or shifting the central electrode.

.With the foregoing and other objects in view, I provide a plasma burnerhaving a ring-shaped electrode located between two annular cylindricalpole shoes having end faces extending forward conically with respect tothe axis thereof, the pole shoes forming a cylindrical chamber wallcarrying excitation windings which are located concentrically about thepole shoes for producing an axial magnetic field. In accordance with theinvention, the plasma burner has a construction wherein an axiallyadjustable central electrode is located in a holder so that it iscoaxial to a cylindrical chamber wall and a single ringshaped electrode,the gas which is to be heated being fed between the holder and thetcylindrical chamber wall.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

While the invention has been illustrated and described as plasma burner,it is not intended to be limited to the details shown since variousmodifications and structural changes may be made therein withoutdeparting from the spirit of the present invention and within the scopeand range of equivalents of the claims.

The invention, however, together with additional objects and advantagesthereof, will be best understood from the following description whenread in connection with the accompanying single figure of the drawingshowing in axial cross section an embodiment of the plasma burnerconstructed in accordance with the invention.

As shown in the figure, a ring-shaped electrode 1 is located between twoannular cylindrical pole shoes 2 and 3. The electrode 1 and pole shoes 2and 3 have a common axis 4. The ends of the pole shoes 2 and 3 adjacentone another extend conically in the direction of the axis 4 toward eachother so as to form an annular, outwardly widening groove between eachother. A central electrode 5 is located centrally to the pole shoes 2and 3 and to the ring-shaped electrode 1. An electric are 6 is formedand extends between the inner periphery of the electrode 1 and the outerperiphery of the central electrode 5 in a direction radial to the axis 4inside the cylindrical arc chamber 7.

The pole shoe 2 is provided on the inner side of the arc chamber 7 witha sleeve 8 of material having a good heat conductivity such as copper,for example. This sleeve 8 is cylindrical and defines with the pole shoe2 an annular slot 9 for coolant. An attached annular cover 10 and athin-walled ring 11 of angular cross section complete the coolantchannel 9. They define the distribution and collection channels orrespectively the outlet and inlet manifolds for the coolant. As shown inthe figure, the tubes 12 supply the coolant, such as water for example,and the tubes 13 remove the coolant.

The pole shoe 3 is essentially mirror-symmetrical to the pole shoe 2. Acoolant channel is formed by an annular slot 9 between a cylindricalsleeve 8 and the pole shoe 3, as well as by a ring 11 of angular crosssection and a ring-shaped member 14a. All connections are effectedeither by soldering or brazing. The coolant is supplied to the slot 9between the sleeve 8 and pole shoe 3 by a tube 12 and is carried away bya tube 13. The ring 11 of angular cross section defines a coolantdistribution channel with a side of the pole shoe which faces the gasflow. When it is sufiicient to cool the pole shoe 3, consisting ofmagnetically conductive material, only in the vicinity of thering-shaped electrode 1, the coolant can then be supplied through thetube 13 and removed through the tube 12, both of which are showndirectly connected to the ring-shaped electrode 1 in the figure.

The magnetic field in the annular slot between the electrodes isconcentrated to angles of inclination of about 60 between the ends ofthe pole shoes and the cylindrical axis due to the conical shape of thepole shoe ends. For the particular ararngement and construction of thepole shoes, reference can be had, moreover, to the experimentalknowledge obtained in the field of electronic or magnetic lenses. It isworthy of note that in the immediate surroundings or vicinity of theconical tips of the pole shoes, the field strength is at its greatest,so that a stabilizing effect is produced for the rotational plane of thearc.

The ring-shaped electrode 1 is radially inwardly, stepwise reduced inthickness and fitted between the shoulders of the ring 11 of angularcross section. The ring-shaped electrode 1 is irremovably grippedthereby. The ringshaped electrode 1 is formed of a non-magneticmaterial, such as copper, which is a good conductor of heat and electriccurrent. It consists of an outer casing 14 having an inner peripheralsurface 15 facing the arc chamber 7 on which the path of the foot orbase of an arc is located. An intermediate member 16, for example anannular disc with a shoulder 17 extends into the casing of thering-shaped electrode 1 so as to form a narrowed cross section of thecoolant channel between the inner peripheral surface 15 and shoulder 17through which the coolant at the ring surface 15 flows with greatvelocity. The ringshaped surface 15 is thus effectively cooled. Asaforementioned, the connecting tubes for supplying and removing coolantare indicated by the reference numerals 12 and 13. If these tubes 12 and13 are formed of a material such as copper, they can also servesimultaneously as electrical connecting leads. The structural componentswith the pole shoes 2 and 3 can be insulated against the ringshapedelectrode 1 by insulating rings such as of ceramic, for example. Insteadof employing rings 11 of angular cross section, the conical ends of thepole shoes can also be provided with a surrounding casing which conformsto the illustrated conical shape. The ring-shaped electrode can also besuitably formed with a reduced portion at the middle of the ring. Such aconstruction has the advantage that the electrodes between the pole shoestructural units remain fixed centrally and immovably. The portion ofthe coolant channel having a greater cross section which is located atthe outer edge of the ring-shaped electrode, acts simultaneously ascoolant distributor or coolant collector channel.

The structural units with the pole shoes 2 or 3 and the ring-shapedelectrode 1 form part of the cylindrical chamber wall of the arc chamber7. With an aligned arrangement of these units a streamlined suitablysmooth channel wall can be obtained. A smooth channel wall is preferablebecause projecting edges are difficult to cool and, furthermore, removea relatively great amount of heat energy from the plasma. The plasmaburner constructed in accordance with the invention has an advantageattributable to the central electrode that the magnetic field isstrongest in the arc foot or base having the widest path. Annularexcitation coils 18 and 19, as Well as 20 and 21, located concentricallyto the pole shoes 2 and 3 respectively, serve to produce the axialmagnetic field. To achieve an axial magnetic field with coils wound inthe same direction, all the annular coils must be connected in the sameway. The excitation windings can be wound of copper tubes through whichthe coolant is conducted. Th electrical sources for exciting theexcitation coils are shown schematically by the batteries 22. In theembodiment illustrated in the figure, the excitation coils 18 and 19, aswell as 20 and 21, are shown as being wound mirror-symmetrical to oneanother so that they therefore are to be connected in the manner shownin the drawing. The excitation windings of the pole shoes are sodistributed that coil units are formed which can be readily cooled.

The magnetic field is conducted through yokes of material having a goodmagnetic conductivity in the magnetic serially arranged pole shoes 2 and3. The excitation windings 18 to 21 can accordingly be mounted within anannular casing 23 which is provided with a slot at the back thereof(shown at the top of the figure) providing access for the connectingtubes 12 and 13, and enclosed within the casing 23 by the annular discs24 and 25. The particular advantage of permitting a coil core field in abeam or bundle in the pole shoes to react on the arc, can be recognizedfrom the following comparison: If one assumes that the inner diameter ofthe arc chamber is mm. and the air gap between the pole shoes is 20 mm.wide, a power loss of only about 6 kilowatts has to be anticipated withthe magnetic coils for a magnetic field of 15,000 gauss, measured at theaxis. If the same field were produced with air core coils alone withoutany iron core, about kilowatt power loss of the coils would have to bereckoned with. In contrast thereto, for a plasma burner wherein an arcextends axially in a cusp field between cylindrical electrodesthe poleshoes 2 and 3 being considered to be electrodes--a coil power loss of3000 kilowatts for a radial field strength of 15,000 gauss has to beproduced in the absence of an iron core. This disadvantage of the cuspfield is due to the fact that only the weaker radial portion of thefield lines acts on the arc. With cusp fields it is also much morediflicult to provide yokes for conducting the field.

A neck 26 is mounted on the ring 1411 of angular cross section on oneside of the arc chamber 7 and serves for guiding the axially adjustablecentral electrode 5. An electrically insulating ring providing agastight seal is located between the ring 14a and the neck 26. Theinsulating ring can be formed, for example, of a plastic material suchas polytetrafiuorethylene. The ring-shaped member 14a of angular crosssection and the neck 26 define an annular space in which the working gasis supplied in a tangential direction through bores 27'. The gas whichis to be heated then flows between the chamber wall and a holder 27 ofthe central electrode with an axial and a radial component. Thus, theworking gas can be supplied in a tangential direction so that the fiowacts opposite to the rotation of the arc 6. The gas will then beconducted in a longer path through the arc and heated especiallystrongly.

The central electrode 5 is made up of a plate-shaped head portion 28having an annular periphery and of the rod-shaped holder 27 which isencased in a plurality of tubes. A dish-shaped part of the head portion28 presenting a concave face to the gas flow maintains the arc foot orbase on an annular path. An outer casing 28 can be formed from glass andserves as a slide tube in the neck 26, which can also be formed ofglass. A casing 29 which can be constructed of a mica base, for example,or other suitable non-conducting material, serves as electricalinsulator. The holder 27 can consist of coaxial tubes which serve ascoolant supply tubes. The coolant is conducted in the dish-shapedelectrode around an annular core 30. Due to the narrow cooling gapadjacent the electrode ring periphery, an efiective coolant action isable to be achieved. The coolant can be supplied to the centralelectrode 5 through a head piece 31 with a distributor channel by meansof a tangential bore 32. The supply voltage for the are 6 is produced bythe schematically shown source 33, the central electrode 5 beingconnected as the cathode and the ring-shaped electrode 1 as anode. Inorder to rigidly secure the central electrode 5 after its adjustment inthe axial direction, as indicated by the double-headed arrow 33', a setscrew can be suitably employed; however, the set screw has been omittedfrom the drawing in the interest of clarity.

The entire plasma burner of the illustrated embodiment can be heldtogether or connected with a few screws indicated by the dot-dash lines34. If the screws 34 at the annular discs 24 and 25 are loosened, on theother hand, the structural units containing the pole shoes can bedisassembled and separated from one another, and the ring-shapedelectrode 1 can be very easily inspected and replaced, if desired.

Insofar as the electrical insulation is exposed to the arc radiation orthe heat rays of the surrounding gas, it can be protected by suitablelabyrinthine construction. Accordingly, the slots in which theinsulating rings are inserted can be covered from view by suitablemeandershaped construction.

If it is desired to provide high flow velocities of the working gas,which can consist of air in the simplest case, a Laval nozzle can beconnected to the open end of the arc chamber. For particularapplications, the excitation windings 18 to 21 can be dispensed with andthe pole shoes 2 and 3 can be formed of permanent magnets. By means of atangential gas supply tube extending in the direction of the arcrotation, the magnetic field can be supported or enhanced.

I claim:

1. Plasma burner for heating a gas to plasma-forming temperature,comprising a pair of coaxially aligned annular cylindrical pole shoeshaving conical end faces extending in the axial direction toward oneanother, electrode means including a ring-shaped electrode locatedbetween said conical end faces, said annular pole shoes and ring-shapedelectrode forming a wall defining a cylindrical chamber, said electrodemeans also including another electrode located in said chamber, andexcitation coils carried by said chamber wall and located concentricallyon said pole shoes, said coils being adapted to produce an axialmagnetic field when electrically energized, said cylindrical walldefining an arc chamber, and said other electrode being axiallyadjustable and being located centrally in said chamber with respect tosaid cylindrical wall, and means for supplying gas to be heated betweensaid central electrode and said cylindrical wall.

2. Plasma burner according to claim 1, wherein said central electrodehas a dish-shaped face located inside an annular rim thereof, and iscarried by an axially adjustable, centrally guided holder member.

References Cited UNITED STATES PATENTS 2,217,187 10/1940 Smith 3131613,201,560 8/1965 Mayo et al 3l3231 X 3,343,019 9/1967 Wolf et a1 313231X JAMES W. LAWRENCE, Primary Examiner R. F. HOSSFELD, Assistant Examiner-U.S. Cl. X.R.

